1. Field of the Invention
The invention relates to a focusing-device for the radiation from a light source, in particular a laser plasma source, having a collector mirror according to the preamble of claim 1 of a type defined more closely.
An illuminating system having a collector mirror is described in U.S. Pat. No. 5,798,823.
2. Description of the Related Art
In semiconductor lithography, use is frequently made of a light source, for example a laser plasma source, whose light is collected, in virtual or real terms, via a collector mirror at a second focus, and then guided for beam formation into an illuminating system. The collector mirror is heated by the laser plasma source, resulting in corresponding changes in shape which have negative effects on the downstream illuminating system such as, for example, illumination defects, for example telecentring errors, uniformity defects, and this can lead to light losses.
It is known for the purpose of avoiding these disadvantages to cool the collector mirror in order to dissipate the heat produced. Irrespective of the large outlay required for this purpose, because of unavoidable tolerances problems continue to exist, nevertheless, with reference to the imaging accuracy, and these are to be ascribed, inter alia, to a change in position of the second focus. In addition, given high thermal loads, which can vary strongly with time in the case of pulsed operation, the collector mirror cannot be kept entirely at a constant temperature level, and so would necessitate a “dynamic” cooling system.